Part 1: Glimpse of EIGRP Routing Protocol

Author : Amandeep Kaur

Today we are going to talk about the one of the dynamic routing protocol named as Enhanced Interior Gateway Routing Protocol (EIGRP). The Question is why and where we are using EIGRP routing protocol and is the need of the EIGRP routing protocol.

First of all, Earlier EIGRP protocol is Cisco propriety protocol and later on it can be used with any vendor router devices. In most of the Networks we saw that OSPF or IS-IS are widely used rather than EIGRP, RIP and IGRP. Yes in some of the cases where the devices are cisco in the networks, the administrators also using EIGRP in the networks.

Thanks for the giving huge support to Routerxp.com, We will come up with many more topics related to routing protocols. Earlier we wrote a article on the EIGRP quick Facts. Below is the link for your reference.

What is the purpose of the EIGRP routing protocol ?
EIGRP routing protocol designed to give all the flexibility of routing protocols with much faster convergence. In addition, EIGRP has Protocol-Dependent Modules that can deal with AppleTalk and IPX as well as IP. The advantage with this is that only one routing process need run instead of a routing process for each of the protocols. 

EIGRP provides loop-free operation and almost instant simultaneous synchronization of all routers. Redistribution between EIGRP and other routing protocols is generally automatic. For example, if IGRP and EIGRP routers use the same AS number then by default routes are redistributed one to the other. 

What about the algorithm used in the EIGRP protocol ?
EIGRP uses the Diffusing Update Algorithm (DUAL) where routers share the route calculations (hence 'diffuse'). A router only sends routing updates as distance vectors of directly connected routes, rather than every route that is in the network. Also, the router only sends an update of a particular if a topology change has occurred to that specific route. 

In addition, this update is only sent to relevant neighbor routers, not to all routers. This makes EIGRP a bandwidth-efficient routing protocol. Other routing protocols have regular routing updates that contain all route information by default. 

Fig 1.1- EIGRP Successor Information table

How it is ensure the packet is delivered smoothly in EIGRP ?
EIGRP packet delivery is handled using Reliable Transport Protocol (RTP) which ensures delivery in order using Reliable Multicast on the multicast address 224.0.0.10. EIGRP uses IP protocol number 88. 

Unlike IGRP, in the IP environment, EIGRP is a Classless routing protocol since updates carry subnet mask information. Although EIGRP automatically summarizes on the network boundary, it can be configured to summarize on any bit boundary. EIGRP can also be used when aggregating routes i.e. when summarizing major networks. 

How many tables are there in the EIGRP ?
EIGRP uses the Neighbor Table to list adjacent routers. The Topology Table lists all the learned routes to a destination whilst the Routing Table contains the best route to a destination, which is known as the Successor. The Feasible Successor is a backup route to a destination which is kept in the Topology Table. 

MD5 authentication can be used to authorize EIGRP packets. 

What Metrics are responsible for the EIGRP path calculation ?
Cisco's EIGRP is similar to IGRP only in the sense that it uses the same metrics; Delay, Bandwidth, Reliability and Load. Be aware that the MTU is NOT used in the calculation of the metric, however the MTU is tracked through the path to find the smallest MTU. 

As with IGRP, the 'K' values for the last three are defaulted to '0'. Only the military use 'Reliability'. Most configurations use the first two metrics Delay and Bandwidth, with Bandwidth taking precedence. The metric for EIGRP is calculated by multiplying the IGRP metric by 256. So the formula used to calculate the metric is 

EIGRP Metric = 256*([K1*Bw + K2*Bw/(256-Load) + K3*Delay]*[K5/(Reliability + K4)]). 

The default values for weights are: 

K1 - 1
K2 - 0
K3 - 1
K4 - 0
K5 - 0 

This gives the default formula of 256*(Bw + Delay) for the EIGRP metric. The term [K5/(Reliability + K4)] is completely ignored if K5 = 0! You can change the weights if you want to. However, just as with IGRP, these weights must be the same on all the routers.

Taking the example we used when looking at IGRP, a link where the bandwidth to a particular destination is 128k and the delay is 84000 microseconds. Using the cut down formula EIGRP metric = 256*(Bw + Delay), we obtain the value 256*(107/128 + 84000/10) which gives 256*86525 = 22150400.